1. Field of the Invention
This invention relates to a torque sensor for detecting a torque generated in a rotating shaft, and more particularly relates to an improved sensor for preventing erroneous detection due to malfunction of a torque detection circuit in a torque sensor provided with a pair of detection coils that change in impedance in the opposite direction of each other correspondingly to the generated torque.
2. Description of Prior Arts
A non-contact type torque sensor has been disclosed in Japanese Published Unexamined Patent Application No. Hei 10-38715 proposed previously by the applicant of the present invention.
In the conventional torque sensor disclosed in the above-mentioned Japanese Published Unexamined Patent Application, the displacement of a torsion bar proportional to the torque is converted into the change of inductance of the detection coil, and the change of inductance is detected by means of a bridge circuit comprising pairs of detection coils and resistors.
In other words, an AC voltage is applied on the bridge circuit provided with first and second arms comprising a pair of detection coils and a pair of resistors, the voltage difference between the output voltage of the junction point of the detection coil and resistor of the first arm and the output voltage of the junction point of the detection coil and resistor of the second arm is detected by means of a differential amplifier, and the torque signal is thereby obtained.
In the torque sensor having the above-mentioned structure, a pair of detection coils and a pair of resistors are connected to a printed wiring board so as to form a bridge circuit. The connection of the detection coil and the printed wiring board is usually secured by soldering. However, possible incomplete connection such as mis-soldering causes the increase of contact resistance between the detection coil and resistor and the printed wiring board, and incorrect torque signal is generated disadvantageously.
Furthermore, the resistance value of the resistor that is a component of the bridge circuit is regarded as a fixed value in the torque detection circuit, but actually the resistance value is changed due to damage of the resistor itself or with the lapse of time.
Such change in the resistance value of the resistor causes the detection circuit to detect it erroneously as if the impedance of the detection coil changes, and as the result an erroneous signal is generated.
Therefore, it has been expected to realize a highly reliable torque sensor that is capable of detecting the erroneous operation when the torque sensor is involved in malfunction.
It is the main object of the present invention to provide a highly reliable non-contact type torque sensor for detecting a torque generated in a rotating shaft that judges the malfunction of a circuit component of the torque detection circuit based on the state of the output signal of the torque detection circuit and prevents the erroneous detection of the torque.
It is another object of the present invention to provide a highly reliable torque sensor for detecting failure of the circuit component of the torque detecting circuit provided with a pair of detection coils having impedances that change in the opposite direction of each other corresponding to the generated torque, in which an AC voltage is applied on a bridge circuit comprising a pair of detection coils and a pair of resistors, the differential voltage between AC voltages that appear on both ends of the pair of detection coils is detected and sent out as the torque signal, and the failure of the circuit component of the torque detecting circuit is judged based on the phase difference between the waveform of the AC voltage applied on the bridge circuit and the waveform of the differential voltage between AC voltages that appear on both ends of the pair of detection coils, and thus the erroneous detection of the torque is prevented.
It is another object of the present invention to provide a highly reliable torque sensor for detecting failure of the circuit component of the torque detecting circuit provided with a pair of detection coils having impedances that change in the opposite direction of each other corresponding to the generated torque, in which an AC voltage is applied on a bridge circuit having a pair of detection coils and a pair of resistors, each resistor comprising two resistor elements connected in series, and the failure of the circuit component of the torque detecting circuit is judged based on the differential signal between the main torque signal that is the differential voltage between the AC voltage that appears on both ends of the pair of detection coils of the bridge circuit and the input AC voltage and the sub torque signal that is the differential voltage between the AC voltage that appears on the middle point between two resistor elements that constitute the pair of resistors of the bridge circuit and the input AC voltage, and thus the erroneous detection of the torque is prevented.
Other objects of the present invention will be apparent from the detailed description of the invention with reference to the drawings.